Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 240, part 1
Prev Page--Hydrology || Next Page--Methodology of Heat-Flow Determination (con't)

Methodology of Heat-flow Determination

Temperature profiles from several boreholes and from different logging studies (table 1) are the basis for the determination of interval temperature gradients and heat flow. Most boreholes were measured in summers of 1980 and 1981 (see Stavnes, 1982; Steeples and Stavnes, 1982) using a thermistor probe with an inherent accuracy of ±0.2°C. Temperatures were obtained at regular depth intervals. The logged intervals range between 1.5 m (4.9 ft) in shallow holes less than 125 m (410 ft), 3.0 m (9.8 ft) in medium and deep holes up to 375 m (1,230 ft), and 4.6 m (15 ft) in air-filled holes. Temperature logs for the boreholes Gibs (identical with well description 1CN2 used by Stavnes), Finegan, Rooks (1RO1 by Stavnes), and Rush (1RH2 by Stavnes) were provided by D. D. Blackwell (personal communication, 1994). Temperatures for these boreholes also were measured with a thermistor probe to the nearest 0.001°C at intervals of 2 m (6.5 ft) and 5 m (16.4 ft). Additional logs were measured by us in February 1994 using the Distributed Optical Fibre Temperature Sensing technique (DTS), which allows a continuous temperature recording along the profile after the installation of the tool (fiber optic cable) in the borehole (for details see Hurtig et al.,1994; Förster et al., 1996). This technique has a resolution of ±0.1°C, precision of about 0.3°C, and a sample interval of 1.0 m (3.3 ft).

Table 1. Boreholes discussed in this paper.

The wells logged by Stavnes had been completed by drilling at least three weeks prior to the logging date to allow temperatures to return to equilibrium. The thermal logs obtained in the boreholes 1CN2/Gibs, Finegan, 1RO1/Rooks by D. D. Blackwell (personal communication, 1994) also probably represent thermal equilibrium because of the lapsed time from drilling; the same is true for the logs we obtained in the boreholes Hodgeman, Finney, and Stanton. The Stanton borehole, however, is very shallow, so the temperature profile may reflect seasonal changes.

The temperature profiles selected for this study show a general increase of temperature with depth. Although some of the wells are constructed as monitoring sites for the Dakota aquifer, hydrological disturbances in the studied boreholes are not substantial nor significant, especially given the long time between the last pumping test and the logging date, which was on the order of years. The problem of intrahole fluid flow is minor because all the boreholes considered in this study had small diameters, 2 or 5 inches (4.1 or 12.7 cm), and were cased.

Lithologic descriptions and stratigraphic details used in interpreting the temperature profiles were taken mostly from sample logs and wire-line logs. Many of the descriptions were either general or nondescript, so that it was not always possible to identify exactly the stratigraphic contacts from descriptions. For example, in places it was difficult to recognize the Graneros-Dakota contact or the Dakota-Kiowa contact, especially in sample logs, because the lithology of both units is similar. Also the lower part of the Carlile is similar to the upper part of the Greenhorn. Therefore, interval temperature gradients were computed from depth intervals with fairly homogeneous lithology, with the assumption that these units also are characterized by a fairly homogeneous thermal conductivity. In some situations, the stratigraphic unit (group) had to be broken into two or more intervals according to lithologic changes. We also have adjusted some of the stratigraphic contacts to better fit the gradient data, which in many instances may be more reliable than the stratigraphic descriptions.

Prev Page--Hydrology || Next Page--Methodology of Heat-Flow Determination (con't)

Kansas Geological Survey
Web version March 14, 1998
http://www.kgs.ku.edu/Current/1997/forster/forster5.html
email:webadmin@kgs.ku.edu